Electroless plating



g- 9, 9 J. s. SALLO 3,265,511

ELEQTROLESS PLATING A Filed June 12'. 1963 No H P02 H O in Gms. Per- Liter INVENTOR.

damm- S. 54440 /ai MA United States Patent The present invention is directed to improvements in the electroless plating of nickel, cobalt .or mixtures thereof.

Electroless plating of nickel and nickel-cobalt alloys has been utilized extensively in recent years due to the unique capabilities of electroless plating not found in ordinary electrolytic plating. For example, electroless plating may be applied directly to nonconductive substrates and may also be used for plating the interior surfaces of parts without the necessity of using the elaborate electroding of electrolytic plating.

In the electroless plating of nickel or nickel-cobalt alloys, a bath is prepared containing the desired metal ion to be plated with a reducing agent such as hypophosphite ion and various additives to provide proper pH and buffering. The reducing agent is capable of reducing the metal ion to the corresponding metal at a catalytic surface. It is well known in the art that these catalytic materials include iron, nickel, cobalt, gold, etc. It is also well known that it is desirable to operate these plating baths at highly elevated temperatures, near the boiling point thereof, in order to obtain a reasonably rapid rate of plating. Further, it is known that higher hypophosphite ion concentration is desirable both to increase the plating rate and to provide a bath which has the ability to plate for a considerable period of time. While these baths have found extensive commercial use, they have also been subject to a recurring problem; the problem of spontaneous decomposition. The present invention provides stabilizing agents for electroless plating paths to inhibit this spontaneous decomposition even when hypophosphite ion concentrations are at levels heretofore unobtainable.

Previous investigators have utilized certain additives in electroless plating baths to provide a degree of bath stability against decomposition. The previously suggested stabilizing agents have been of the sulfur containing type such as thiourea. The mechanism in this type of stabilizing action is the coating of surfaces of finely divided particles of metal, which occur spontaneously in the bulk of the bath, with some product of the sulfur bearing compound so as to inhibit the minute metal particle from reacting further with the bath in the manner of the catalytic surface.

The present invention is directed to providing stabilty against the homogeneous decomposition of the bath. By homogeneous decomposition is meant the formation of free metal within the body of the bath. Thus, the present invention is directed to the prevention of the formation of the minute particles of metal rather than the inhibition of further reaction with these particles of metal.

While the exact mechanism by which the present invention operates is not clearly known, it appears that the oxidizing agents of the present invention combine with an intermediary product of the reaction of the hypophosphite ion with the bath constituents. This intermediate product, it is postulated, reacts with nickel ion in the bulk of the solution to produce nickel metal. The present invention provides an agent within the bath which will react with this intermediate product in the bulk of the bath and prevents it from reacting with nickel or other metal ion being plated so as to provide a nucleation site in the bulk of the bath.

Accordingly, it is an object of the present invention to provide stabilizing agents for electroless plating baths;

It is a further object to provide stabilizing agents which will permit increased concentration of reducing agents without spontaneous decompositions;

Other and further objects will be apparent from a study of the specification and drawings wherein:

FIGURE 1 is a plot of hypophosphite ion concentration vs. plating rate for a plating bath in accordance with the present invention.

The improvement in plating bath stability made possible by the present invention is shown in the following examples. These baths are simply illustrative of the operation of the additives of the present invention. Numerous changes may be made in composition insofar as concentration, choice of buffering or complexing agents and the like. For discussion of these factors, the reader is directed to Metal Finishing magazine for June, July, August, and September of 1962 for an extensive discussion and bibliography of electroless plating.

Example I An electroless nickel plating bath having the following composition was formulated.

A bath formulated as above and heated to 205 F. without the presence of a catalytic surface would decompose uncontrollably within approximately 15 minutes. That is, minute particles of nickel metal appear spontaneously in the bulk of the solution and then, being catalytic themselves, act as nucleation points to decompose the bath. Detection of decomposition in a hath not containing a catalytic surface may be readily discerned by the appearance of bubble formation and gas evolution.

However, when an addition is made to the plating solution of an oxidizing agent in accordance with the present invention, the plating bath remains completely stable for 60 minutes and shows only slight evidence of decomposition after this time. Oxidizing agents showing the capability for increasing plating bath stability have an E.M.F. of from 0.6 to 1.2 v. in the solution. The E.M.F. or E of the additive is calculated from the Nernst equation wherein it is assumed that the reaction has proceeded 10 percent of the way from the oxidized form of the oxidizing agent to the reduced form. referred to is according to Latimer.

Sign convention The quantity of oxidizing additive used in this general case was 8X10' mols per liter. The following partial listing is illustrative of additives of the invention falling within this range.

Nitrate with an E of 0.55 volt and Br with an .E of 1.45 volts have no effect as stabilizers.

It is of interest to note that hydrogen peroxide will also provide the improved stability of the other oxidizing agents listed above. Apparently, the hydrogen peroxide will decompose to give free oxygen, and the free oxygen then reacts with some intermediary product of the hypophosphite reduction to provide stability in the same manner as described in my copending application, Serial Number 245,234, filed Dec. 17, 1962, entitled Electroless Plating and assigned to the same assignee as the present invention.

Example II A plating bath in accordance with Example I was prepared with the exception that the concentration of the oxidizing agents of the present invention were increased to l6 l0 mols per liter. The first evidence of even slight decomposition (some evolution of gas from the bath) occurred after 75 minutes.

Increasing the concentration of oxidizing agent above these levels (or below) will produce the expected results (decrease in bath stability with decreasing concentration of additive and increasing stability with increased concentration).

Example III As a further example of advantages to be gained through the use of my invention reference is made to FIGURE 1 wherein there is shown a plot of relative plating rate v. concentration of hypophosphite ion. The bath composition was the same as that in FIGURE 1 with the exception that hypophosphite ion concentration was varied. Ferric chloride was utilized as the addition agent in concentration of 8 l0 mols per liter. As can be seen plating rate is dependent on hypophosphite ion concentration up to approximately 36 gms. per liter. Without the use of ferric chloride the plating bath would spontaneously decompose more and more rapidly as hypophosphite ion concentration is increased over the 24 grams per liter of Example I. By the use of the present invention additives, very high concentrations of hypophosphite ion concentration are made possible. For example, a plating bath in accordance with Example I containing 36 gms./l. of hypophosphite ion Without the presence of an oxidizing agent of the present invention would spontaneously decompose almost immediately on reaching a temperature of 190 F. With the use of an oxidizing agent such as ferric chloride the same bath will show prolonged stability at 205 F. As is known, the concentration of nickel ions in a bath of this type has only minor influence on stability. The concentration may vary over wide limits.

Example IV The present invention also will prove useful in mixed electroless plating baths of the nickel-cobalt type. Nickel cobalt electroless plated deposits have found use as magnetic memory elements. However, in the desired bath composition difficulty due to spontaneous decomposition has made these baths a problem to the user. The tendency to spontaneously decompose is dependent on the ratio of nickel to cobalt and is most pronounced in the range of 40 to 90 weight percent nickel of the total nickel-cobalt present in the plating bath. A typical plating bath of this type for use in producing nickel-cobalt deposits useful as magnetic memory elements is:

NiCl 6-H O gm./l 30.0 CoCl 6H O 33.3 NaH PO H O gm./l 83.3 NH Ol g m./l 33.3 Sodium citrate gm./l T-hiourea .cg./l ph 7.8

A bath of the above composition is quite unstable at elevated temperatures due to the high hypophosphite ion concentration. However, by use of oxidizing agents as described above in Example I the tendency of this bath to spontaneously decompose is markedly reduced. The stabilizing agents which prove effective are those having an E of 0.6 to 1.2 volts. The following oxidizing agents are effective stabilizers when used in concentration as in Examples I and II.

Agent E (in volts) Fe+++ 0.77 I 0.9 Br, 1.2 ClO 0 67 Cr O 0 62 N0 0 64 As in Example I hydrogen peroxide also proves to be an effective stabilizing agent.

As was the case in the limits of E in Example I NO with an E of 0.50 and MHO4 with an E=1.20 are not eflective for stabilization.

The above examples have illustrated the invention used in electroless plating baths having marked instability. It should be realized that even in those electroless plating baths which are normally more stable than those of the above examples that the present invention will prove useful to inhibit the spontaneous decomposition which also occurs in the normally stable baths.

Having thus described my invention, I claim:

1. In an electroless plating bath of the type wherein hypophosphite ion is utilized to reduce metal ions of the class consisting of nickel and mixed nickel cobalt to metal form on a catalytic surface the improvement which comprises including within said bath in a quantity from about 8X10 up to about 16 10" mols/liter an oxidizing agent selected from the group having an electromotive force in solution calculated from the Nernst equation when the reaction has proceeded 10 percent of the way from oxidized to reduced form of from greater than 0.6 to less than 1.2 volts and hydrogen peroxide.

2. In an electroless nickel plating bath wherein hypophosphite ion is utilized to reduce nickel ions to nickel metal at a catalyticsurface the improvement which comprises including within said bath in a quantity from about 8 10 up to about 16 10 mols/liter of an oxidizing agent selected from the group having an electromotive force in solution calculated from the Nernst equation when the reaction has proceeded 10 percent of the way from oxidized to reduced form of from greater than 0.6 to less than 1.20 volts and hydrogen peroxide.

3. In an electroless plating bath of the type wherein hypophosphite ion is utilized to reduce metal ions of the class consisting of nickel and mixed nickel cobalt to the metal form on a catalytic surface the improvement which comprises including Within said bath a quantity of hydrogen peroxide in an amount from about 8 to about 16 X l0 mols/ liter.

4. In an electroless plating bath of the type wherein hypohposphite ion is utilized to reduce metal ions of the class consisting of nickel and mixed nickel cobalt to the metal form on a catalytic surface the improvement which comprises including within said bath a quantity of from about 8 l0- up to about 16 l0- mols/liter of an 3,265, 5 1 1 5 6 oxidizing agent selected from the group having an electro- References Cited by the Examiner motive force in solution calculated from the Nernst equation when the reaction has proceeded 10 percent of the UNITED STATES PATENTS Way from oxidized to reduced form of from greater than 2,029,386 2/1936 Pine 204-1 0.6 to less than 1.20 volts. 5 2,955,944 10/1960 Spaulding 106-1 5. In an electroless plating bath of the type wherein 2,994,369 8/1961 Carlin hypophosphite ion is utilized to reduce metal ions of the t class consisting of nickel and mixed nickel cobalt to the ALEXANDER BRODMERKEL Primary Examinen metal form on a catalytic surface the improvement which comprises including within said bath a quantity of Fe+++ 10 MORRIS LIEBMAN, Examiner- '4 fi g ig from 8X10 4 to about 16x10 J. E. CARSON, L. HAYES, Assistant Examiners, 

1. IN A ELECTROLESS PLATING BATH OF THE TYPE WHEREIN HYPOPHOSITE ION IS UTILIZED TO REDUCE METAL IONS OF THE CLASS CONSISTING OF NICKEL AND MIXED NICKEL COBALT TO METAL FORM ON A CATALYTIC SURFACE AND IMPROVEMENT WHICH COMPRISES INCLUDING WITHIN SAID BATH IN A QUANTITY FROM ABOUT 8X10**-4 UP TO ABOUT 16X10**-4 MOLS/LITER N OXIDIZING AGENT SELECTED FROM THE GROUP HAVING AN ELECTROMOTIVE FORCE IN SOLUTION CALCULATED FROM THE NERNST EQUATION WHEN THE REACTION HAS PROCEEDED 10 PERCENT OF THE WAY FROM OXIDIZED TO REDUCED FORM OF FROM GREATER THAN 0.6 TO LESS THAN 1.2 VOLTS AND HYDROGEN PERIOXIDE. 